The present invention relates to the field of techniques for ground improvement and deep foundations.
In general manner, ground improvement techniques seek to consolidate terrains that present heterogeneous structure, in particular when they are unsuitable for building.
Such techniques include making a mesh of rigid structures in ground, commonly known as rigid inclusions. These structures are made to improve the load-bearing capacity of the ground and to reduce settlement.
More precisely, the present invention relates to a machine for making rigid structures in ground, and to a method using said machine.
The invention is particularly suitable for making such structures having low top levels.
A structure is generally said to have a low top level when the top end of a structure is several meters below the working platform.
Presently known techniques for making such structures generally provide for a continuous column to be made up to the level of the working platform and then for the column to be struck off down to the depth desired for its top level, e.g. using a mechanical digger when the material is still fresh, or by destroying it after the material has hardened (e.g. using a pneumatic drill, by splintering, or chemically).
Those various techniques involve working in a plurality of stages, thereby lengthening time to completion. They also require various different tools to be used. Striking-off when the material has hardened also presents problems associated with health and safety for operators (noise and vibration when using pneumatic drills, risk of receiving splashes with chemical methods). Striking off fresh material involves making large-sized excavations that destroy the ground and destabilize the work platform.
French patent application FR 2 960 571 in the name of the Applicant discloses a machine making it possible with a single tool and in a single stage to make a mixed-material or “hybrid” column comprising a bottom portion forming a rigid structure and a top portion made of ballast. That machine comprises a ground perforation tool and a tank arranged around the perforation tool in order to be inserted into the ground by being vibrated, either by vibrating the perforation tool to which it is coupled, or by the action of an independent vibrator. Concrete is introduced into the ground by the perforation tool over a first length that is to form the bottom portion of the column, and then ballast is poured into the ground from the tank while the tank is being raised.
Tests have shown that that machine is not suitable for making structures with low top levels, since the tank cannot be inserted to a sufficient depth in all terrains.
It has also been found that vibrating the tank has a harmful effect on the mast, since the combined vibration of the perforation tool and of the tank, acting respectively on the high and low portions of the mast weaken the machine considerably.